In recent years, along with the development of digital technology, functions of electronic devices such as portable information devices and information home appliances have been further improved. As these electronic devices become more sophisticated in functionality, finely structured, high speed semiconductor elements to be used in the electronic devices have been rapidly developed. Among all, application of a nonvolatile memory having a large capacity represented by the flash memory has been rapidly expanding. In addition, research and development of Resistive Random Access Memory (ReRAM) having a variable resistance element as a new next-generation nonvolatile memory to replace the flash memory has advanced. Here, a variable resistance element is an element which has characteristics that a resistance value reversibly changes according to an electrical signal, and which can store information corresponding to the resistance value in a nonvolatile manner.
The resistive random access memory uses a variable resistance layer having a variable resistance value as a memory element. Application of an electrical pulse (for example, a voltage pulse) to the variable resistance layer causes the resistance value to change from a high resistance state to a low resistance state, or from a low resistance state to a high resistance state. In this manner, the resistive random access memory stores data. In the above process, it is necessary to clearly distinguish two values in a low resistance state and a high resistance state, to cause a change between a low resistance state and a high resistance state stably and quickly, and to hold the two values in a nonvolatile manner.
As an example of such a nonvolatile memory element, a nonvolatile memory element using a variable resistance layer in which transition metal oxides having different oxygen content atomic percentages are stacked has been proposed. For example, PTL 1 discloses a technology which stabilizes resistance change phenomenon by selectively causing an oxidation reaction or a reduction reaction at an electrode interface to come into contact with a transition metal oxide layer having a high oxygen content atomic percentage.
FIG. 23 is a cross-sectional view illustrating a variable resistance nonvolatile memory device 50 having a nonvolatile memory element 55 described in PTL 1. In the nonvolatile memory device 50 illustrated in FIG. 23, a first line 61 is formed on a substrate 60, and a first interlayer insulating layer 62 is formed to cover the first line 61. In addition, a first plug 64 connected to the first wiring 61 is formed so as to penetrate through the first interlayer insulating layer 62. Furthermore, the nonvolatile memory element 55 is formed on the first interlayer insulating layer 62 so as to cover the first plug 64. The nonvolatile memory element 55 comprises a lower electrode 65, a variable resistance layer 66, and an upper electrode 67. A second interlayer insulating layer 68 is formed so as to cover the nonvolatile memory element 55. A second plug 70 is formed so as to penetrate through the second interlayer insulating layer 68. The second plug 70 connects between the upper electrode 67 and a second line 71.
The variable resistance layer 66 has a stacked structure including a first variable resistance layer 66x and a second variable resistance layer 66y. The first variable resistance layer 66x and the second variable resistance layer 66y comprise transition metal oxide of the same type. The oxygen content atomic percentage of the transition metal oxide comprised by the second variable resistance layer 66y is higher than the oxygen content atomic percentage of the transition metal oxide comprised by the first variable resistance layer 66x. 
With the above-described structure, when a voltage is applied to the nonvolatile memory element 55, most of the voltage is applied to the second variable resistance layer 66y having a high oxygen content atomic percentage and exhibiting a higher resistance value. Oxygen contributing to a reaction is abundantly present in the vicinity of the second variable resistance layer 66y. Thus, an oxidation reaction or a reduction reaction selectively occurs in the vicinity of the interface between the upper electrode 67 and the second variable resistance layer 66y, therefore, a resistance change can be stably achieved.
NPL 1 discloses a nonvolatile memory comprising a 1T1R (1 transistor 1 resistance) memory cell which uses transition metal oxide as a variable resistance layer. A transition metal oxide thin film is normally an insulating material. Thus, in order to change a resistance value with a pulse, the variable resistance layer is broken in an initial state (initial break), thereby forming a conductive path having a switchable resistance value between a high resistance state and a low resistance state. It is to be noted that “initial break (initial breakdown)” is a process which is performed on a variable resistance layer after manufacture to make a transition to a state in which the variable resistance layer can reversibly change between a high resistance state and a low resistance state according to a voltage value applied (or the polarity of a voltage applied). Specifically, the initial breakdown is to apply a voltage (initial breakdown voltage) higher than a write voltage to a variable resistance layer having an extremely high resistance value after manufacture or to a nonvolatile memory element including the variable resistance layer. The initial breakdown causes the variable resistance layer to make a transition to a state in which the variable resistance layer can reversibly change between a high resistance state and a low resistance state, while the resistance value of the variable resistance layer is reduced.